They would quit because they would see the number of trophies people are racking up compared to themselves. If you've played 17 hours this week thinking you were doing well, and see that you have 1/9th the trophies of the person at the top, you'd do some quick math and realize that there was no way you should bother and put in that kind of time. The groups create the illusion that you're doing well and should continue to play.
Ok, I see what you mean.
I do think a totally ignorant person, who has only read the rules, still has a chance. They would simply choose to play in arenas for 16 hours a day (something like that). Such a person would have already gotten (or come very close to) 10k trophies in a single week, yet we haven't seen anyone like that yet.
In other words a totally naive person giving max effort would be able to top the leaderboards, so they shouldn't give up hope yet.
(They would give up when they realize a 3000 bullet player giving max effort would be close or more than 20k in a week, but that takes a little bit of investigating.)
Maybe I have to explain the arena thing (?)
In arena, you get bonus trophies equal to arena points.
Arena points are awarded for wins, and win streaks.
A win is worth 2, but your second consecutive win is worth 3. Consecutive wins after that are all worth 4.
For example 2 wins in a row = 5 arena points
3 wins in a row = 9
5 wins in a row = 17
The way arenas are scheduled, you play about 12-15 games. So someone who wins a blitz arena may have won 15 games. Someone who wins a bullet arena may have won 15 games.
So far nothing sounds unfair... but the point is bullet arenas are 30 minutes long while blitz are 1 hour long. This means I can play two bullet arenas for every blitz arena. Doubling the arena points means doubling the bonus trophies.